Small objects tend to evolve over short timescales. For example, in chemistry, biology, and materials science, a sequence of changes no more than a nanometer in size can pass by in a microsecond, leaving behind little evidence. Reconstructing these processes and determining how and why they took place can be a difficult task. Many techniques can record the static, before and after states of materials, some even at high spatial resolutions, but they often lack the temporal resolution to capture the nearly instantaneous changes occurring in between.
For example, conventional transmission electron microscopy/microscopes (TEM) is a well-established technique/instrument using electrons instead of light for investigating material properties and structure at size scales from micrometers to angstroms at the atomic level. TEMs operate on the same principles as a light microscope but substitute electrons for light to achieve much higher spatial resolution. Electron microscopes typically generate electrons through thermionic emission, in much the same way an incandescent bulb uses heat to produce light, or through field emission, which combines a metallic conductor and an electrostatic field. Electrons emitted by a source, usually at the top of the microscope, are typically focused by magnetic lenses into a narrow beam and directed through a thin specimen. Depending on the specimen's material properties, for example its density and crystalline structure, some of the electrons are scattered and used to form an image or diffraction pattern. Subsequent lenses in the TEM column magnify this image or pattern onto a fluorescent screen. The resulting light and dark regions provide information about the materials examined, including their crystalline grain structure, dislocations, or even single atomic rows and columns. A camera at the bottom of the microscope then records the data. Conventional TEMs produce a steady stream of electrons that pass through the optical column one at a time, and achieve the desired beam parameters using small apertures with lenses to discard all but a fraction of the current.
Despite the relatively high spatial resolutions (i.e. spatial resolutions higher than light microscopes) achievable by conventional TEM microscopes, their relatively low temporal resolutions (e.g. second- or millisecond-scale) are often inadequate to capture the fast dynamic processes in materials, e.g. microstructured or nanostructured materials, which occur at such small size scales. In particular, conventional TEM measurements are made at roughly standard video frame rates (e.g. ˜24 frames per second), which are governed by the electron source current and brightness and the capabilities of the image acquisition systems, and which are often thousands or even millions of times slower than the rate at which processes typically evolve in microstructured or nanostructured materials. Such material processes also are often unique, never unfolding exactly the same way twice, necessitating single-shot high-speed acquisition for their study. Examples include mechanical deformation and the interaction between phase transformations and microstructure. To capture dynamic behavior with a conventional TEM microscope, researchers must start and stop a process, which is not always feasible and rarely precise.
Dynamic transmission electron microscopy (DTEM) is another methodology similar to and modified from conventional TEM technology for investigating material properties and structure. A microscope making use of DTEM technology, however, can capture transient processes/events in materials with enhanced (e.g. nanosecond-scale) time resolution as a high quality image or diffraction pattern of the state of a material at some known time interval after an event has begun. In contrast to conventional TEM instruments, which produce a steady stream of electrons (such as produced by thermionic emission) that pass through the optical column one at a time, a DTEM instrument releases electrons in a single burst as an extremely brief (e.g. 1 ns to few microseconds timescales) bright electron pulse operating at high electron current, producing billions of electrons in a pulse. The high current is typically achieved through photoemission, driven by a pulsed laser (e.g. ultraviolet laser) directed upon a metal cathode, such as for example a tantalum disk. In particular, the laser is arranged to direct a pulse of light into the optical column of the TEM where it may be reflected by a mirror onto the cathode to release a burst of electrons. The electron pulse emitted from the electron source is then accelerated through a system of condenser lenses that focus and point the beam upon the investigated sample. It is notable that while conventional TEMs achieve the desired beam parameters using small apertures with lenses to discard all but a fraction of the current, the lens system of a DTEM microscope is typically designed with additional condenser lenses and an extra focusing (crossover) region, and also with a reduced number of apertures, to maximize electron throughput while focusing the beam down to a small spot on the sample to boost spatial resolution. Furthermore, the duration of the laser pulse determines the “exposure time” for recording the image or diffraction pattern.
In this manner, a DTEM microscope controls the electron emission such that it is correlated in time with the transient process being studied, which is typically triggered or initiated by a pulsed laser striking the material, though other methods are possible. As a result, a DTEM microscope produces a brighter, higher current beam than a conventional TEM with little sacrifice in beam coherence (which affects image contrast and diffraction-pattern sharpness). In addition, researchers can adjust the lens's focal length to control how much of the beam is used in an experiment. Because a DTEM microscope acquires sufficient information in a single experiment to generate a high-resolution image of a micro- to nanosecond-long, nanoscale event, irreversible events such as phase transformations, chemical reactions and crystal growth can be studied.
Unfortunately, standard DTEM microscopes are configured for single shot operation to only capture a single image or diffraction pattern per sample drive event/camera read-out time, which is typically on the scale of milliseconds to seconds. Thus many of the details of complex, unique processes may still be lost. Single-shot DTEM experiments are typically repeated on fresh specimens as often as required, each with a different time delay, and the collection of images are combined into an averaged view of the process over time. While an averaged view is sufficient for studying reactions that are nearly identical every time, and while a high time-resolution single image provides some research value, they lack the ability to scrutinize more complex or variable behavior and fast material processes ranging from phase transformations to chemical reactions and nanostructure growth.